Superchargers are used to increase engine horsepower by forcing more air-fuel mixture into cylinders of an automotive engine. A normal engine depends upon the vacuum created by pistons to draw the mixture into the cylinders. The supercharger assists the vacuum by increasing the pressure of the incoming air. One known type of control system for such superchargers has the feature of feedback control to develop the pressure of incoming supercharging air to a predetermined target pressure of supercharging air. Such a supercharge pressure control system is known, for example, from Japanese Unexamined Patent Publication No. 61(1986) - 16240 entitled "Supercharge Pressure Control System For Automotive Engine With Supercharger" laid open Jan. 24, 1986.
In the supercharge pressure control system described in the above publication, the same control gain of feedback control is used independent from whether the pressure of actual or incoming supercharging air is higher or lower than a predetermined target pressure of supercharging air. Generally, the control gain of feedback control is substantially always small for the reason of preventing the automotive engine from hunting over the whole range of engine operating conditions in order to ensure the operating stability of an automotive engine.
One disadvantage associated with an automotive engine utilizing the supercharge pressure control system in which feedback control is always effected with the same control gain is that, when the pressure of incoming supercharging air is higher than the target pressure of supercharging air, the automotive engine is subjected to, and thereby adversely affected in the durability thereof by, a great load for a long time.